Marine propulsion systems generally comprise outboard motors or stern drive systems which transmit rotary power to a propeller to drive a boat through water. The propeller includes propeller blades which are angled to provide propulsion through the water. The angle or pitch of the blades relative to a radial axis transverse to the drive axis of the propeller is generally fixed and selected to provide maximum efficiency at maximum speed or cruise speed of the boat to which the system is used. The pitch is generally less efficient at take-off when the boat is driven from stationary up to the cruise speed, which inefficiency results in increased fuel consumption and a longer time for the boat, to move from the stationary to cruise speed. If the propeller has too large pitch, the power of the engine may not be sufficient to accelerate the boat to planing speed.
In order to overcome this problem, variable pitch propeller systems have been proposed in which the pitch of the propeller blades can be altered to suit the changing operating conditions of the propulsion system. Our International Application No. PCT/AU99/00276 discloses such a system which is particularly suitable for outboard motor applications.
Pitch control systems which are used in stern drives generally comprise hydraulic systems for adjusting the propeller pitch and are therefore relatively expensive and complicated. The size of such systems can also be of issue because it is generally desired that the drive system be as small as possible to minimise drag through the water and weight of the system.
As a consequence, conventional systems are generally not suitable for retrofit to existing stern drives.
Controllable pitch systems also suffer from the problem that if the system breaks down, it is possible that the pitch of the propeller blades will be in a position where it makes emergency propulsion of the boat impossible so that the boat cannot be driven by the propulsion system even if the motor is operable to rotate the propeller.
Furtherstill, the fact that the propeller blades are adjustable in pitch means that the propeller hub is generally complicated and includes a number of parts which usually include bevel gear arrangements. Such arrangements have been found to allow some oscillation of the propeller blades around their fixed position which can significantly impair operation of the propeller in some operating conditions.